The long range goal of this work is to improve failure-free survival in children receiving conventional dose cyclophosphamide (CY) by personalizing cancer treatment and/or CY doses, either by pharmacogenetics (Aim 1) or by pharmacokinetics (Aim 2). The majority of pediatric cancer patients receive CY as a key component of their initial curative combination chemotherapy regimens. The current method of dosing CY by body surface area or weight leads to considerable interpatient variability in the systemic exposure, expressed as area under the plasma concentration-time curve (AUC), of CY and its metabolites. Variability in the exposure to 4-hydroxycyclophosphamide (4HCY), the principal precursor to the cytotoxic metabolite of CY, may account for interpatient differences in the efficacy of CY. Our working hypothesis is that polymorphisms of enzymes and transporters involved in the production and elimination of 4-hydroxycyclophosphamide (4HCY), the principal precursor to the cytotoxic metabolite of CY, are associated with a higher ratio of 4HCY/CY area under the curve (AUC) and improved survival; moreover, this association is dependent on age and CY dose. In Aim 1, we will test the hypothesis that genetic polymorphisms of the drug metabolizing enzymes and transporters involved in the production and elimination of 4HCY are associated with failure-free survival to vincristine/actinomycin/CY (VAC) in children with newly diagnosed intermediate risk rhabdomyosarcoma. This aim will be completed as a correlative study to a phase III Children's Oncology Group trial. Furthermore, we overcame a critical barrier to understanding CY pharmacology in children by being the first group to characterize the ratio of 4HCY/CY AUC in this population. Our data indicate this ratio is higher and more variable in children less than 10 years old than in older children and adults. In addition, the ratio of 4HCY/CY AUC is lower after myeloablative dose CY (60 mg/kg) relative to conventional dose CY (0.4-1.2 g/m2). Thus, in Aim 2, we will establish the need for and feasibility of pharmacokinetic adjusted dose strategies in children receiving conventional dose CY. We will develop more sensitive analytic methods to quantitate CY, 4HCY, and other CY metabolites, while also simplifying our bedside processing method to stabilize 4HCY in plasma. We will also create a population pharmacokinetics-based limited sampling schedule. These tools will decrease the amount of blood needed to characterize CY pharmacokinetics, allowing us to characterize within-patient variability and include infants. Expansion of our pharmacokinetic cohort will allow for a better understanding of which factors - both genetic (using the candidate genes of Aim 1) and non-genetic (e.g., age, CY dose) - are associated with an aberrant ratio of 4HCY/CY AUC. This proposal fits well within the major focus of the NCI Program Announcement (PA-086025), as this grant proposes to conduct ancillary clinical studies that propose improvements and evaluations of CY-based combinatorial treatments and patients' prognoses.